High-rate hydrogenotrophic denitrification in a pressurized reactor

Most conventional hydrogenotrophic denitrification reactors based on packed- or fluidized-bed present a similar H₂ delivery scheme of continuous gas purging to the atmosphere in order to improve H₂ transfer rates and enable discharge of N₂ gas produced during denitrification. This operation results in a significant release of H₂ gas to atmosphere with its related economic and safety concerns. The current research proposes a novel pressurized high-rate hydrogenotrophic reactor for denitrification without gas purging. The investigation performed refutes a prevalent notion that N₂ gas accumulates in the headspace of a closed reactor during denitrification. Instead, this research shows that during continuous operation a gas-liquid equilibrium is established in the reactor according to Henry's law and excess N₂ gas is carried out by the effluent in dissolved form. Therefore, no gas purging is required and H₂ loss is limited only to the dissolved H₂ in the effluent. As a consequence, a simple low-cost and high-rate reactor with closed headspace can be designed for denitrification. The proposed reactor is operated as a trickling filter where water is recirculated over biofilm carriers with high surface area.The feasibility of the proposed reactor was shown for two effluent concentrations of 10 and 1mgNO₃^--N/L. Average denitrification rates of 2.1±0.2 and 1.06±0.06gNO₃^--N/(L_reactord) with H₂ utilization efficiencies of 92.8% and 96.9% were measured for the two effluent concentrations, respectively. Higher denitrification rates of up to 5gNO₃^--N/(L_reactord) were observed at higher recirculation flow rates and higher partial pressures of H₂.